Well, only some of them actually offer full VPN service. Most of them are still just traffic-forwarding proxies, just not limited to HTTP. NordVPN used to offer full VPN service under the name "Meshnet", but actually discontinued it last year.

All of them offer only proxied access to the internet. They do not expose access to any "private network".

> You can use them for whatever protocol you want.

the two most commons protocols used for proxying traffic support arbitrary tcp traffic. socks is quite self explanatory but http is not limited to https either!

Of course most providers might block non https traffic by doing DPI or (more realistically) refusing to proxy ports other than 80/443 but nothing is inherent to the protocol.

edit: this is also mentioned on MDN: https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/...

> Aside from enabling secure access to websites behind proxies, a HTTP tunnel provides a way to allow traffic that would otherwise be restricted (SSH or FTP) over the HTTP(S) protocol.

> If you are running a proxy that supports CONNECT, restrict its use to a set of known ports or a configurable list of safe request targets

> A loosely-configured proxy may be abused to forward traffic such as SMTP to relay spam email, for example.